Transcript What_is_splicing

What is RNA splicing?
Genetic information is transferred from genes to the
proteins they encode via a “messenger” RNA
intermediate
DNA
GENE
transcription
messenger RNA
(mRNA)
translation
protein
Most genes have their protein-coding information
interrupted by non-coding sequences called “introns”. The
coding sequences are then called “exons”
DNA
exon 1 intron exon 2
GE
NE
transcription
precursor-mRNA
(pre-mRNA)
intron
The intron is also present in the RNA copy of the gene
and must be removed by a process called “RNA
splicing”
pre-mRNA
intron
RNA splicing
mRNA
translation
protein
Splicing a pre-mRNA involves two reactions
intron branchpoint
pre-mRNA
A
Step 1
intermediates
A
Step 2
spliced mRNA
Splicing occurs in a “spliceosome”
an RNA-protein complex
(simplified)
spliceosome
(~100 proteins + 5 small RNAs)
pre-mRNA
spliced mRNA
Splicing works similarly in different organisms, for
example in yeast, flies, worms, plants and animals.
RNA is produced in the nucleus of the cell. The
mRNA has to be transported to the cytoplasm to
produce proteins
Ribosomes are RNA-protein machines that make
proteins, translating the coding information in the
mRNA
Pre-messenger RNA Processing
pre-mRNA
M7G
exon
intron
exon
AAAAAAA200
cap
RNA splicing
mRNA
M7G
nucleus
AAAAAAA200
transport
cytoplasm
M7G
AAAAAAA200
ribosomes
protein
poly(A) tail
Alternative splicing
In humans, many genes contain multiple introns
intron 1
1
intron 3
intron 2
2
3
1
2
3
intron 4
4
4
5
Usually all introns must be removed before the
mRNA can be translated to produce protein
5
However, multiple introns may be spliced
differently in different circumstances, for
example in different tissues.
Heart muscle mRNA
pre-mRNA
1
1
2
3
2
Uterine muscle mRNA
5
3
1
3
4
4
5
5
Thus one gene can encode more than one protein. The proteins are
similar but not identical and may have distinct properties. This is
important in complex organisms
Different signals in the pre-mRNA and different proteins
cause spliceosomes to form in particular positions to give
alternative splicing
We are studying how mRNAs and proteins interact in
order to understand how these machines work in general
and, in particular, how RNA splicing is regulated as it
affects which proteins are produced in each cell and
tissue in the body.
Alternative splicing can generate mRNAs encoding proteins with
different, even opposite functions
Fas ligand
Fas
5 6 7
(membraneassociated)
Fas pre-mRNA
5
(+)
6
7
APOPTOSIS (programmed
cell death)
(-)
Fas ligand
5 7
Soluble Fas
(membrane)
Alternative splicing can generate tens of thousands of mRNAs
from a single primary transcript
Combinatorial selection of one exon at each of four variable regions generates more than
38,000 different mRNAs and proteins in the Drosophila cell adhesion molecule Dscam
12
48
33
2
pre-mRNA
mRNA
protein
The protein variants are important for wiring of the nervous system and for immune response
Examples of the potential consequences of mutations on splicing
A
Mutations occur
on the DNA
(in a gene)
1
2
no mutation
normal mRNA
1
2 3
4
normal protein
active
B
3
mutation A
truncated mRNA
5
C
1
2
truncated protein
inactive
4
5
mutation B
exon 3 skipped
1
2
4
mutation C
longer exon 4
5
1
2 3
4
protein of different size (smaller or longer)
inactive or aberrant function
5
Pathologies resulting from aberrant splicing can be
grouped in two major categories
 Mutations affecting proteins that are involved in splicing
Examples:
Spinal Muscular Atrophy
Retinitis Pigmentosa
Myotonic Dystrophy
 Mutations affecting a specific messenger RNA and disturbing its
normal splicing pattern
Examples:
ß-Thalassemia
Duchenne Muscular Dystrophy
Cystic Fibrosis
Frasier Syndrome
Frontotemporal Dementia and Parkinsonism
Therefore, understanding the mechanism of RNA
splicing in normal cells and how it is regulated in
different tissues and at different stages of
development of an organism is essential in order to
develop strategies to correct aberrant splicing in
human pathologies